Crustal structure of Guadeloupe islands and the Lesser ... - CiteSeerX

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Bull. Soc. géol. France, 2013, t. 184, n 1-2, pp. 77-97

Crustal structure of Guadeloupe islands and the Lesser Antilles arc from a new gravity and magnetic synthesis LYDIE-SARAH GAILLER1, GUILLAUME MARTELET1, ISABELLE THINON1, VINCENT BOUCHOT1, JEAN-FRÉDÉRIC LEBRUN2 and PHILIPPE MÜNCH3, 4 Key-words. – Lesser Antilles arc, Guadeloupe island, Geophysical surveys, Geophysical data synthesis, Magnetic and gravity anomalies, Modeling, Seismic profiles, Subsidence, Deformation, Crustal structures

Abstract. – Guadeloupe island (West French Indies) is one of the twenty islands that compose the Lesser Antilles arc, which results from the subduction of the Atlantic ocean plate beneath the Caribbean one. The island lies in a complex volcano-tectonic system and the need to understand its geological context has led to numerous on- and offshore geophysical investigations. This work presents a compilation and the processing of available, on-land, airborne and marine, gravity and magnetic data acquired during the last 40 years on Guadeloupe islands and at the scale of the Lesser Antilles arc. The overall dataset provides new Bouguer and reduced to the pole magnetic anomaly maps at the highest achievable resolution. Regionally, the main central negative gravity trend of the arc allows defining two subsident areas. The first one is parallel to the arc direction (~N160oE) to the north, whereas the second unexpected southern one is oriented parallel to oceanic ridges (N130oE). Along the Outer arc, the long wavelength positive anomaly is interpreted, at least along the Karukera spur, as an up-rise of the volcanic basement in agreement with the seismic studies. To the NE of Guadeloupe, the detailed analysis of the geophysical anomalies outlines a series of structural discontinuities consistent with the main bathymetric morphologies, and in continuity of the main fault systems already reported in this area. Based on geophysical evidences, this large scale deformation and faulting of the Outer arc presumably primarily affects the Atlantic subducting plate and secondarily deforms the upper Caribbean plate and the accretion prism, as evidenced in bathymetry as well as on the islands. At the scale of Guadeloupe island, combined gravity and magnetic modeling has been initiated based on existing interpretation of old seismic refraction profiles, with a general structure in three main layers. According to our geophysical anomalies, additional local structures are also modeled in agreement with geological observations: i) the gravity and magnetic signals confirm an up-rise of the volcanic basement below the limestone platforms outcropping on Grande-Terre island ; ii) the ancient volcanic complexes of Basse-Terre island are modeled with high density and reverse magnetized formations; iii) the recent volcanic centre is associated with formations consistent with the low measured density and the underlying hydrothermal system. The E-W models coherently image a NNW-SSE depression structure in half-graben beneath Basse-Terre island, its western scarp following the arc direction in agreement with bathymetric and seismic studies to the north of the island. The so-defined depressed area, and particularly its opening in half-graben toward the SW, is interpreted as the present-day front of deformation of the upper plate, associated with the recent volcanic activity on and around Guadeloupe. Based on this regional deformation model, perspectives are given for further integrated investigation of key targets to address the internal structure and evolution of the Lesser Antilles arc and Guadeloupe volcanic system.

Structure crustale des îles de Guadeloupe et de l’arc des Petites Antilles d’après une nouvelle synthèse gravimétrique et magnétique Mots-clés. – Arc des Petites Antilles, Guadeloupe, Synthèse géophysique, Anomalies gravimétriques et magnétiques, Modélisations, Profils sismiques, Subsidence, Déformation, Structures crustales.

Résumé. – La Guadeloupe (Antilles françaises) est l’une des vingt îles qui composent l’arc des Petites Antilles, résultant de la subduction de la plaque Atlantique sous la plaque Caraïbe. Elle s’intègre dans un système volcano-tectonique complexe et la nécessité de comprendre son contexte géologique a conduit à de nombreuses investigations géophysiques à terre comme en mer. Cette étude présente la compilation et le traitement des données gravimétriques et magnétiques terrestres, aéroportées et marines acquises ces 40 dernières années sur les îles de Guadeloupe, mais également, à l’échelle de l’arc des Petites Antilles. Le jeu de données ainsi compilé a permi de dériver des cartes d’anomalies de Bouguer et d’anomalies magnétiques réduites au pôle à la plus haute résolution possible. Régionalement, l’anomalie gravimétrique négative centrale de l’arc suggère deux zones de subsidence: l’une parallèle à l’arc (N160oE), et l’autre plus au sud de direction N130oE. Le long de l’arc externe, nous interprétons l’anomalie gravimétrique positive de grande longueur d’onde en termes de bombement du socle volcanique, notamment au niveau de l’éperon de Karukera. L’analyse détaillée des anomalies géophysiques met en évidence un éventail de discontinuités géophysiques cohérentes avec les principaux reliefs bathymétriques dans l’alignement de systèmes de failles déjà reconnus dans ce secteur. Des arguments géophysiques indiquent que ces zones de déformations et de fracturations majeures affectent en premier lieu

1. BRGM, 3 Avenue Claude Guillemin, 45060 Orleans Cedex 02, France. [email protected] 2. Université des Antilles et de la Guyane, EA 4098 LaRGE, 97159 Pointe-à-Pitre, Guadeloupe, FWI 3. Université Montpellier 2, UMR 5243 Géosciences Montpellier, place E. Bataillon, 34095 Montpellier Cedex 05, France 4. Université de Provence, 3 place Victor Hugo, 13331 Marseille Cedex 3, France Manuscript received on October 10, 2011; accepted on April 4, 2012 Bull. Soc. géol. Fr., 2013, no 1-2

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la plaque Atlantique plongeante et se répercutent en surface sur la plaque Caraïbe et le prisme d’accrétion. A l’échelle de l’île de Guadeloupe, des modélisations gravimétriques et magnétiques conjointes sont initiées sur la base des interprétations antérieures de profils sismiques réfraction à trois couches majeures: i) les signaux gravimétriques et magnétiques modélisés confirment la présence de remontées de socle volcanique à l’aplomb des plateformes carbonatées affleurantes sur Grande-Terre ; ii) les complexes volcaniques anciens de Basse-Terre correspondent à des formations denses et inversement aimantées ; iii) le centre volcanique récent est associé à des formations cohérentes avec les densités mesurées et le système hydrothermal sous-jacent respectivement. Les coupes gravimétriques et magnétiques d’orientation E-W imagent, à l’aplomb de Basse-Terre, une dépression en demi-graben dont la bordure ouest est parallèle à la direction de l’arc. Ces modélisations s’accordent avec les études bathymétriques et sismiques en mer, mais également géologiques à terre. Nous interprétons sa bordure SW en demi-graben comme le front actuel de déformation de la plaque Caraïbe, lié à la subduction. Cette zone est également associée au volcanisme actif de l’île de Guadeloupe. Le modèle régional ainsi établi appelle à de nouvelles investigations détaillées, dans le but de mieux contraindre la structure interne, ainsi que l’évolution de l’arc et du système volcanique de Guadeloupe.

INTRODUCTION Volcanic islands often have a complex geological history of volcanic and tectonic events. Particularly for subduction islands, this history is difficult to unravel, both on-land, because of the resurfacing by subsequent activity, erosion and vegetal coverage, and offshore, where geological information is difficult to access. The knowledge of the internal structure of such volcanic systems relies on geophysical investigations providing images of the spatial distribution of physical parameters, which can then be interpreted in terms of geological structure and evolution. In this paper we will focus on the Lesser Antilles arc and in more detail on Guadeloupe archipelago in order to better understand how this island is integrated in its regional context. Since the 1970’s, a substantial amount of work has been carried out to study the islands of Guadeloupe [e.g. Coron et al., 1975; LeBorgne and LeMouël, 1976; Dorel et al., 1979; Le Mouël et al., 1979; Grellet et al., 1988; Gunawan, 2005; Calcagno et al., 2011; Mathieu et al., 2011; Lardeaux et al., 2013; Münch et al., 2013] and also the submarine domain of the overall Lesser Antilles arc [e.g. Boynton et al., 1979; Feuillet, 2000; Deplus et al., 2001; Feuillet et al., 2001; Guennoc et al., 2001; Truffert et al., 2004; Thinon et al., 2010; Kopp et al., 2011; Lebrun, 2011; Münch et al., 2013]. We present here the compilation of all gravity and magnetic data available from old and recent surveys over Guadeloupe archipelago and more regionally at the scale of the Lesser Antilles arc. The methods we have used provide different types of information. First, due to faults commonly juxtaposing rocks of different densities, the gravity method is often a powerful tool for identifying the structural setting of volcanic areas. The submarine flanks materials could sometimes be differentiated and characterized due to their density contrast. Secondly, the interpretation of the magnetic signal is effective for distinguishing the constructed structures, which retain a coherent remanent magnetization, from the ones formed by breccias (debris avalanches…), which exhibit only a weaker induced magnetization. In addition, because Guadeloupe volcanism encompasses at least one major magnetic reversal (Brunhes-Matuyama at 0.78 Ma), the analysis of the magnetic anomalies discriminates areas predominantly composed of formations either younger or older than the reversal. This synthesis provides a new approach for studying the 3D structure of the Guadeloupe island in its regional Lesser Bull. Soc. géol. Fr., 2013, no 1-2

Antilles volcanic setting. In such geodynamic context, the strong volcano-tectonic control is also investigated in order to understand the structure and evolution of the Guadeloupe complex as a whole, in the frame of the volcanic arc setting. GEODYNAMIC CONTEXT AND GEOPHYSICAL STUDIES Geodynamic context The Lesser Antilles arc is a 850 km long archipelago which results from the subduction of the Atlantic ocean plate beneath the Caribbean one, at a rate of around 2 cm.yr-1 in a southwestern direction [Bouysse, 1979; Deng and Sykes, 1995; Dixon et al., 1998; DeMets et al., 2000]. Eastern of the accretion prism, the Atlantic oceanic plate is highly fractured and includes N130 oE ridges (fig. 1a). To the north, the Barracuda and Tiburon ridges present a morphologic expression, whereas to the south, the Ste Lucia ridge is completely buried under the sediment cover and only detected in seismic reflection profiles [Andreieff et al., 1987]. At the scale of the Caribbean plate, the arc, convex towards the east, extends from 12 oN to 18oN on the northeastern edge of the lithospheric plate, with 20 islands including the French Lesser Antilles. North of the Martinique island, the Lesser Antilles diverge into two sub-arcs, the outer and the inner northern arcs (fig. 1a). Guadeloupe islands (fig. 1b) are part of both of these sub-arcs. It is composed, to the east, of Grande-Terre, La Désirade, Petite-Terre and Marie-Galante islands, which belong to the Limestone Caribbean from the early pre-Miocene outer arc. These islands are uniform, reef limestone platforms, developed during the early Pliocene-middle Pleistocene interval [Andreieff et al., 1987; Bouysse et al., 1993; Lardeaux et al., 2013] which cover the Eocene-Miocene volcanic basement [Bouysse, 1979; Le Mouël et al., 1979; Bouysse et al., 1988; Bouysse et al., 1993]. To the west, Guadeloupe consists of Basse-Terre and Les Saintes islands which belong to the recent volcanic inner arc [Andreieff et al., 1987]. In more details, the western volcanic island of Basse-Terre consists of five eruptive complexes that were emplaced from north to south, along a direction of around N160oE, at different periods over the past 2.8 Ma [Samper et al., 2007]. They are composed of an assemblage of composite volcanoes developed in an active sinistral transtensional fault zone, the Montserrat-Bouillante-Les Saintes fault system (MBS; fig. 1a [Feuillet et al., 2001; Feuillet et al., 2010; Thinon

CRUSTAL STRUCTURE OF GUADELOUPE ISLANDS AND THE LESSER ANTILLES ARC

et al., 2010; Calcagno et al., 2011]). The northern part of Basse-Terre includes the oldest volcanic complexes (Basal complex: 2.79-2.68 Ma, and Septentrional chain: 1.8-1.15 Ma; fig. 2). The younger volcanic complexes, La GrandeDécouverte, Madeleine-Soufrière, was developed around 0.2 Ma (fig. 2) ago to present in the central part of the island between the Monts Caraïbes (0.5-0.4 Ma; fig. 2) and the Axial chain (2.5-0.6 Ma; fig. 2; ages from [Samper et al., 2007]). To the west, along the Bouillante coast, the volcanic episodes of the Bouillante chain are thought to have been active between 1.2 and 0.9 Ma (fig. 2) [Briden et al., 1979], later revised between 0.8 and 0.2 Ma [Blanc, 1983; Carlut et al., 2000]. Geophysical studies Bathymetric studies The bathymetry around Guadeloupe island remained poorly known until 1998 when the Aguadomar survey [Deplus et al., 2001] has provided detailed bathymetry for a large part of the Lesser Antilles arc. More recently, the BATHYBOU98 (1998) survey [Guennoc et al., 2001], the Kashallow2

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[Lebrun et al., 2009] and Kashallow 3 [Lebrun, 2011] surveys have acquired high resolution bathymetric data in the offshore environment of Guadeloupe archipelago. The compilation and interpretation of these high-resolution bathymetric data have been carried out by different authors (Deplus, IPGP: Institut de Physique du Globe de Paris; Lebrun, UAG: Université Antilles Guyane; Guennoc and Gailler, BRGM: Bureau de Recherche Géologique et Minière, SHOM: Service Hydrographique et Océanographique de la Marine). The areas not covered by these surveys have been constrained using the gebco bathymetric data (http://www.gebco.net/). Although completed with data from various resolutions, our compilation is sufficient to provide the basis for a detailed image of the submarine domain of Guadeloupe island but also at larger scale (fig. 1). We have computed a digital elevation model (DEM) of the Lesser Antilles arc, with a mesh cell size of 50 m covering an area of about 112,500 km2 from Barbados to the north to St Lucia to the south. It highlights the different types of submarine domains of the arc: shelves, the slopes cross-cut by the submarine canyons, the deep abyssal plains, debris avalanches and sedimentary deposits, oceanic ridges and submarine volcanoes.

FIG. 1. – a) Location of Guadeloupe archipelago in the Lesser Antilles arc, and location of the main regional faults (MBS: Montserrat-Bouillante-Les Saintes fault system, BC: Bouillante-Capesterre fault, MG: Marie-Galante basin, modified from [Bouysse et al., 1983; Feuillet et al., 2001; Thinon et al., 2010; Calcagno et al., 2011]). Location of the Barracuda, Tiburon and Ste Lucia ridges from Andreieff et al. [1987]; b) Shaded relief map of Guadeloupe islands (framed in a) showing the main places discussed in text. Compilation of the bathymetric data: this study; data source: DEM Kashallow3 (UAG, J.-F. Lebrun), DEM Aguadomar (IPGP, C. Deplus), Hydrographic probes of SHOM, DEM BATHYBOU98 (BRGM, P. Guennoc), and Gebco model. Bull. Soc. géol. Fr., 2013, no 1-2

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Gravity and magnetic studies

GEOPHYSICAL DATA PROCESSING

Several on-land gravity surveys were carried out since the 1960’s on Guadeloupe Islands by groups from various French organizations (BRGM, BGI, and IPGP). The first Bouguer anomaly map of Guadeloupe islands has been published by Coron et al. [1975] who have provided general interpretations of the gravity anomalies. A more detailed survey which focuses on the Bouillante geothermal province has been carried out in 1984 by the BRGM [BRGM, 1984]. More recently, Gunawan [2005] has proposed an interpretation of La Soufrière gravity structure using a composite gravity map. In this study, we have compiled all these on-land data in order to investigate in detail the gravity structure of the archipelago. The gravity stations are distributed as shown in figure 3a with a heterogeneous density of data, at a spacing of 200 m to 1.5 km in average. The final database is composed of 1229 land gravity stations (tab. I).

Gravity data

An aeromagnetic survey of the overall archipelago was carried out in 1975, at an altitude of 500 m above sea level over Grande-Terre and 1800 m over Basse-Terre [Le Borgne and Le Mouël, 1976]. The two panels overlap along a band of 10 km covering the emerged relief and extending from 10 to 30 km beyond the island coast (fig. 3b). The flight lines are oriented W-E and spaced 10 km apart, with a distance of about 80 m between successive measurements, and an uncertainty on the horizontal coordinates of about 100 m [Le Borgne and Le Mouël, 1976]. First interpretations of the resulting anomaly map were published by Le Mouël et al. [1979], allowing to precise or evidence some tectonic discontinuities, but also to highlight the relationship between the main onshore and offshore structures. Offshore, the new gravity and magnetic data acquired during the Aguadomar survey (1998) have provided homogeneous and good quality gravity and magnetic coverage of the submarine flanks at the scale of the Lesser Antilles arc. In addition, the Geoberyx (2003) and Kashallow cruises (2009 and 2011) have also provided high resolution magnetic data of the submarine flanks of Guadeloupe islands (fig. 2c and d). Good quality magnetic data from a transit route of the Arcante cruise (fig. 2d) carried out in 1980 have also been included in our dataset. For the offshore gravity data, we have also compiled the data from Move survey (fig. 2c) carried out in 2002 to the east of the area of interest. These latter are available from the NGDC database (http://www.ngdc.noaa.gov/).

At the time of our study, the on-land gravity data were already reduced using standard procedures, namely tidal variations and instrumental drift. We have used the 1980 international gravity formula to determine the theoretical gravity at each station location on the Earth’s spheroid. The Bouguer slab correction has been computed considering the Earth’s curvature [Lafehr, 1991]. For the marine data, we have first decimated the different datasets in order to reduce the navigation noise and to homogenize the data sampling between the considered surveys. We have neglected the drift correction, but an intersection adjustment has been performed at profile crossings, for each survey first, and then between the different surveys, considering Aguadomar cruise as the reference for the overall dataset (tab. I). Due to the common strong topography of volcanic edifices, terrain effects are significant on Guadeloupe island. A general assumption in determining the optimal density correction is to consider the value minimizing the correlation between the gravity anomaly and the elevation [Nettleton, 1939]. A series of anomaly maps were computed for different density correction values ranging from 1.6 to 3.2 10 3 kg m-3 in order to determine the minimum correlation between gravity and topography at the scale of the overall edifice. These maps are displayed in supplementary data with a large scale Nettleton profile crossing the Guadeloupe island and its submarine environment. Especially over BasseTerre, on-land gravity anomalies and the topography appear correlated for density values ranging from 1.6 to 2.4 103 kg m-3, and reversely correlated for values exceeding 2.8 103 kg m-3. A density value of 2.67 103 kg m-3 minimizes best the anomaly-topography correlation along the volcanic edifice. Even if the submarine correlation reaches its minimum for slightly lower densities, a homogeneous reduction density of 2.67 103 kg m-3 has been applied to the whole map in order to make the interpretation more straightforward. We have corrected the topographic effects using Oasis Montaj 7.2. A far zone correction is computed using a coarse regional digital elevation model (DEM), whereas the near zone correction is computed with a more detailed local one. The near zone is a 5×5 km square centered on the station and the far zone extends from 5 to 167 km. The on-land data have been corrected using a 25 m DEM from the Institut Géographique National (IGN) for the near zone and

TABLE I. – Summary of the gravity and magnetic data used to compile the new Bouguer and magnetic anomaly maps.

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a 250 m DEM for the far zone. For the offshore data, because data were measured farther from the bathymetry, we have used a 500 m DEM for the near zone and a 1000 m DEM for the regional corrections. The density of sea water was assumed to be 1.027 103 kg m-3. After interpolation, a very low amplitude and high frequency noise was often noted on the marine gravity data, and was filtered out using a low-pass filter with 800 m cutoff wavelength. In a conventional interpolation, short-wavelength, elongated anomalies intersected by several measurement profiles are usually rendered as strings of ellipsoidal beads along the measurement profiles. These types of effects are commonly minimized using filtering and gridding along trend directions. We used the ’Trend Enforcement algorithm’ from Oasis Montaj, which is designated to provide a solution which preserves the character of local trends while eliminating aliasing effects. This process results in smoother maps with better defined anomaly trends. In addition, an obvious very long-wavelength regional trend oriented NW-SE, which is parallel to the arc alignment and can be interpreted as being produced by deep crustal structures. In order to focus on the local subsurface structures, this regional low frequency has been estimated by a low-pass butterworth filter with a 200 km cutoff wavelength (fig. 4b). The so-derived gravity surface allows well reproducing the two main long wavelength anomalies of the original signal: the negative one to the NW of the map as well as the eastern linear positive one elongated following the arc orientation. This regional also takes into account, the positive trend which appears, at least in relative to the SW of the map, western of La Martinique and Dominique islands. This latter was subtracted from the original gravity map (fig. 4a). The resulting residual map represents the local Bouguer anomaly (fig. 4c) computed with 71,702 data for a mesh cell size of 800 m (tab. I).

acquired at an altitude of 500 m were upward continued to the highest elevation of acquisition, i.e. 1800 m. Following the same rational, the offshore magnetic data, acquired at sea level, were upward continued at the same elevation (1800 m), providing a homogeneous magnetic anomaly map (fig. 5a) of the data available at the scale of the Lesser Antilles arc computed with a 91,209 data for a mesh cell size of 800 m (tab. I). The last processing applied to the magnetic anomaly map is the reduction to the pole (RTP; fig. 4b), which is intended to relocate magnetic anomalies on top of their causative sources as far as magnetization is induced [Baranov, 1957]. This transformation required the direction of the apparent magnetization and that of the ambient field to be known. The apparent (or total) magnetization corresponds to the sum of the remanent and induced magnetizations. The induced magnetization vector is collinear with the ambient field (present magnetic field vector in Guadeloupe: declination ~ –0.14 o; inclination ~ 43.92o), whereas the remanent magnetization is the combination of various types of magnetizations. The generally high Koenigsberger ratio (i. e. the ratio of the remanent magnetization to the induced magnetization) of volcanic rocks indicates a preponderance of the thermoremanent magnetization (TRM) component over the induced one. Although the present magnetic field differs significantly from that of a geocentric axial dipole field for that latitude (declination of 0o and inclination of 29.7 o), we have used this direction for the magnetization in the RTP transformation, since it represents well the TRM direction of the terrains when secular variation effects are averaged.

Magnetic data

Regional anomalies of the Lesser Antilles arc (fig. 4c)

The on-land and marine magnetic data have been reduced using standard procedures, including the correction of the distance between the magnetometer and the ship or the aircraft, the intersection adjustments, and the IGRF (International Geomagnetism Reference Field) reduction. This last operation allows correcting the data from the normal magnetic field at the place and time of acquisition when the surveys are carried out at different epochs. The resulting anomalies are therefore coherent within and between the considered surveys. As in the case of the marine gravity data, the marine magnetic data have been decimated in order to reduce the navigation induced noise and to homogenize the data density between the different considered surveys. An intersection adjustment has been also performed for both aeromagnetic and marine magnetic profile crossings. For the offshore data, this processing has been firstly done for each survey, and then between the different surveys considering Aguadomar cruise as the reference for the overall dataset (tab. I). As previously mentioned, the aeromagnetic data have been acquired at two different elevations. Because of the elevation of the highest subaerial summits, it is difficult to make a downward continuation of the aeromagnetic data. In order to merge both sets of aeromagnetic data, the data

To the first order, the gravity signal of the Lesser Antilles arc can be divided in three large domains globally oriented N160oE, i.e. parallel to the arc direction:

QUALITATIVE INTERPRETATION OF THE NEWLY COMPILED GEOPHYSICAL DATA Bouguer anomaly map

FIG. 2. – Time relations between geomagnetic epochs and Guadeloupe island formations. Dating from Samper et al. [2007]. Bull. Soc. géol. Fr., 2013, no 1-2

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– the eastern domain is marked by a large elongated positive anomaly oriented NNW-SSE along the Outer arc, and superimposed to the accretion prism. This anomaly was

first identified by Coron et al. [1975] or Dercourt [2002] and interpreted by Gunawan [2005] as crustal thickening related to the subduction of an oceanic plate;

FIG. 3a. – Distribution of the gravity stations available at the scale of the Guadeloupe islands.

FIG. 3b. – Distribution of the aeromagnetic data over the Guadeloupe islands.

FIG. 3c. – Distribution of the marine gravity profiles available at the scale of the Lesser Antilles arc.

FIG. 3d. – Distribution of the marine magnetic profiles available at the scale of the Lesser Antilles arc.

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– the western domain; limited to the west by the MBS fault system, is marked by a lower amplitude positive anomaly associated with the Inner arc. The gravity lineaments coincide with the bathymetric relief in this area; – the central area, associated with the emerged relief axis between the ancient and recent volcanic arc systems, is marked by a NNW-SSE curved negative anomaly also parallel to the arc from Antigua island to the north to La Martinique island to the south. These three main domains are clearly dissected by shorter wavelength anomalies. The gravity scheme is globally more complex to the north of the study area, and appears less contrasted to the south of Guadeloupe archipelago. The large eastern positive gravity signal is subdivided in three main long wavelength positive anomalies in the offshore continuation of Antigua, Guadeloupe and Martinique islands. These latter are separated by negative or slightly positive minima oriented about N065 oE. The northern positive bloc (to the east of Antigua) presents a main orientation of N150 oE, and is limited to the northeast by a negative WNW-ESE trend coincident with the Barracuda ridge. The central block is oriented N170oE, and limited to the north by La Désirade valley and separated to the south by a N130oE axis parallel to Tiburon ridge. This block includes the Karukera spur which was recognized as the accretion prism crustal backstop that rises about 4000 m above the fore-arc basin [De Min et al., 2011]. According to a seismic tomography work carried out at the scale of the central Lesser Antilles island arc, Kopp et al. [2011] have evidenced a decrease of the sediment cover thickness in the central area along the spur. In parallel, complementary seismic studies [Evain et al., 2011; Ruiz et al., 2011] have evidenced two units within the fore-arc: the Outer fore-arc associated with the accretion prism to the east and the Inner fore-arc to the west including Karukera spur (fig. 4c). This latter, also associated with our main central positive gravity block, has been interpreted as a crustal buldge just in front of the fore-arc outer basin. It appears tilted as a whole to the south, since the Karukera block is larger and more developed to the south than its bathymetric expression and is deeply buried beneath sediments [De Min et al., 2011; Evain et al., 2011]. It could result from an original oceanic crust thickened by deep magmatic processes and underplating prior to the evolution of the Lesser Antilles arc [Dieblod, 2009]. Within the southern block, the observed main positive gravity anomaly N-S oriented is cross-cut by several structures oriented N130oE and seems to be limited to the south by the St Lucia ridge. Accordingly, to the east of La Dominique island, the gravity lows are elongated in the same N130 oE direction. The westernmost domain is globally positive and smoothly structured by an alternation of slightly positive and negative anomalies elongated following a N060 oE orientation, i.e. perpendicular to the arc direction. These alternations presumably reflect the signature of large debris flow slumping from the islands within the western valleys [Le Friant et al., 2002; Le Friant and Boudon, 2003; Le Friant et al., 2004]. The southwestern coast of Basse-Terre is marked by a well-defined elongated negative anomaly striking the N140 o-N160oE direction along the MBS major sinistral transtensional fault system [Calcagno et al., 2011; Mathieu et al., 2011]. This main anomaly integrates the submarine volcanic edifices in Bouillante area and seems to

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continue southwards, with an apparent inflexion south of Les Saintes (from N160oE to about N140 oE), consistent with the already reported MBS fault system inflexion [Feuillet et al., 2002; Thinon et al., 2010] The main central negative anomaly forms a well-defined trend all along the ancient arc direction. To the north, its western border corresponds to the MBS fault with a N160oE orientation, and continues southwards, east of Dominique and Martinique islands. The northeastern submarine area of Guadeloupe, Marie-Galante basin and La Désirade valley are also marked by strong negative anomalies. Another distinct negative trend strikes N130 oE along the northern coast of Dominique island, toward the SE, parallel to the oceanic ridges, i.e. the Barracuda, Tiburon and St Lucia ridges. This last anomaly joins the southern end of the MBS and thus defines a curvilinear negative area, which cross-cuts and structures the southern positive block of the eastern domain. During the Arcante survey (1980) [Andreieff et al., 1987] some reef limestones from the middle Oligocene to early Miocene were dredged along the western flank of the La Désirade valley at a depth of about 950 m and along the southern flank of Karukera spur at a depth of about 1600 m. In parallel, marls from the same epoch were found at about 650 m b.s.l. to the south of Petite-Terre island, in Marie Galante valley. Such old carbonated formations recognized in deep waters in the central part of the arc argue for a subsident zone active at least since the middle Oligocene-early Miocene [Andreieff et al., 1987; De Min et al., 2011]. On-land, the gravity signal over Marie-Galante displays a positive gradation from north to south which is related to the relief and the fault system which cross-cuts the island in the E-W direction. It could be interpreted as a slightly higher level of volcanic basement to the south of Marie Galante as proposed for Karukera spur by Kopp et al. [2011]. Around Les Saintes, the sparse data do not allow defining precise anomalies. At least, the old volcanic edifices seem to be associated with relatively more negative anomalies.

FIG. 4. – a) Bouguer anomaly map computed for a density correction of 2.67×103 kg m-3; b) regional anomaly obtained with a 200 km low-pass butterworth filter. Bull. Soc. géol. Fr., 2013, no 1-2

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La Désirade is included within the main positive signal of the eastern domain to the north of Karukera spur. The island exhibits the oldest rocks of the Lesser Antilles arc and the Carribean plate, which are interpreted in terms of an old volcanic basement outcropping to the east of the island [Corsini et al., 2011]. Since it is located in the northwestern part of the inner fore-arc [Evain et al., 2011], our gravity map is also consistent with an up-rise of the volcanic basement just beneath the island. However, no specific shallow anomaly could be detected at this resolution.

locally evidenced a NW-SE structure cross-cutting the volcanic dome, which may be associated with deeper structures, in agreement with the regional trend observed both on-land and offshore [Feuillet et al., 2001; Thinon et al., 2010; Mathieu et al., 2011]. The eastern coast of Basse-Terre (including the erosional plain) is associated with intermediate low amplitude gravity signal, N-S oriented, parallel to the isthmus between Basse-Terre and Grande-Terre islands. In this area, the existence of N-S oriented faults connecting to the main E-W extensional structures have been reported [Terrier, 2010].

The Guadeloupe s.s. anomalies (fig. 4d) As already mentioned by Coron et al. [1975], the highest amplitude Bouguer anomalies are centered on the old edifices to the north (Basal complex and Septentrional chain). On the contrary, the youngest volcanic centers are characterized by lower anomalies. Basse-Terre island can be divided in three main contrasted gravity domains (fig. 4d). The NW area (Basal complex and Septentrional chain) is associated with positive anomalies which could be related to the andesitic lava flows emitted by the associated complexes [Westercamp and Tazieff, 1980]. To the SW, the coastal area associated with the Bouillante chain and the western parts of the actual active complex are well marked by positive anomalies. In the southern continuation, the Monts Caraïbes correspond to a lower amplitude positive anomaly. On the contrary, the active central area (La Soufrière-Grande Découverte) is clearly marked by a strong negative anomaly in the continuation of the negative signal of the Axial chain. Gunawan [2005] has

Grande-Terre Island appears more homogeneous from a gravity point of view, with an overall general negative Bouguer anomaly (fig. 4d) in agreement with the smooth relief and the limestone nature of the outcropping formations. At least in relative, three main positive anomalies contrast in this area: to the west, in the Grands Fonds zone were the tables are dissected by a network of valleys, and pitted by numerous karstic dollines [Feuillet, 2000; Léticée, 2008]; to the north, in the area of Saint Jacques plateau dissected by a series of large N050oE normal fault scarps [Feuillet, 2000], and to the southeast, in the vicinity of Saint-François. From the Grippons Plain graben to Saint-François, it is noticeable that main gravity lineaments are parallel to the N130oE major structures that crosscut the island [Münch et al., 2013]. These anomalies could tentatively be explained by heterogeneities of the limestone platform, but are more probably related to an up-rise of the underlying basement as previously described for Karukera spur [Kopp et al., 2011] and Marie Galante island.

FIG. 4c. – Residual gravity map obtained by subtracting the regional anomaly from the observed one; this residual anomaly emphasis the short-wavelength signal. The main regional faults are located, MBS: Montserrat-Bouillante-Les Saintes fault system; BC: Bouillante-Capesterre fault. Bull. Soc. géol. Fr., 2013, no 1-2

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FIG. 4d. – Extract of the residual gravity map centered on the Guadeloupe archipelago. The main places discussed in text are located.

RTP magnetic anomaly map Regional anomalies of the Lesser Antilles arc (fig. 5b) As previously mentioned by Le Mouël et al. [1979], the submarine environment of the Lesser Antilles arc is dominated by high amplitude and long wavelength negative anomalies. To the first order, the recent arc is associated with a negative magnetic trend globally oriented N170 oE and aligned with the MBS fault system in its central part. This high amplitude negative magnetic trend continues to the south along the western coast of Dominique and Martinique islands. Based upon the amplitude and the wavelengths of the associated anomalies, several magnetic domains could be outlined. As for the gravity map, the magnetic scheme appears more complex in the northern part of the study area. The northwestern area mostly displays lower amplitude and mainly negative magnetic en-echelon lineaments. To the NE, several elongated high amplitude anomalies appear, from Marie Galante island toward the north, as compared to the southern part of the arc which is associated with less and larger anomalies. In more detail, the northeastern area is dominated by the juxtaposition of both positive and negative en echelon trends. According to the World digital magnetic anomaly map (http://models.geomag.us/wdnam/html) measured at 5 km above sea level, these latter seem to continue further to the north along the N-S main bathymetric directions of the arc (not presented here). To the north of the Outer arc, they define irregular anomalies globally N-S oriented. For example, the N-S positive anomaly which

extents from Marie Galante to Petite-Terre islands seems to be shifted to the west of La Désirade island in the western continuation of the Tiburon ridge. The spatial distribution of these N-S anomalies suggests that they might result from the deformation of the plate along five main structural axes ranging from about N020o to N130oE. These shifts are also observed in the extension of Marie Galante valley, Antigua valley and Bertrand Falmouth spur. These fan-shaped structures are coherent with the seabed morphology of the Outer arc mapped by Feuillet et al. [2010]. To the SE and SW of the map, long wavelength positive anomalies appear to the west of Dominique island and to the east of Martinique island. These N-S magnetic anomalies are difficult to interpret, since their smoothness argues for a deep origin. More locally, Marie Galante basin is associated with a large negative anomaly (fig. 5 b and c). On the contrary, the onland island magnetic signal is marked by a strong positive anomaly extending offshore, and elongated in the N-S direction. This latter has been interpreted by Le Mouël et al. [1979] as the trace in surface of a large intrusive dome in this area. On the magnetic map computed at the altitude of acquisition in this area (500 m, not presented here), i.e. more detailed, the eastern part of the island is associated with a strong positive short wavelength signal. This observation argues for a shallower source and suggests that Marie Galante anomaly can be at least partly related to an up-rise of the volcanic basement centered beneath its eastern part [Bouysse and Garrabé, 1984]. Bull. Soc. géol. Fr., 2013, no 1-2

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Around Les Saintes, keeping in mind the rather low resolution of our anomaly map measured at an altitude of 1800 m, the islands are associated with a negative signal, except a small positive spot which could be associated either to normally magnetized volcanic products (i.e. recent), or to a local basement high, as already proposed for Marie Galante island. Finally, La Désirade is associated with a positive N-S oriented anomaly in the eastern continuation of the previously described deep-seated N-S magnetic lineament. A parallel but shorter wavelength positive anomaly appears to the east of the island, with the same distribution underlined on the map computed at 500 m a.s.l. As previously mentioned, since oceanic crust outcrops in the east of La Désirade [Corsini et al., 2011], this is also consistent with an up-rise of the volcanic basement just beneath the eastern part of the island. The Guadeloupe s.s. anomalies (fig. 5c) To the north of Basse-Terre island, the old Basal complex is associated with a positive magnetic anomaly consistent with the age of the associated formations [Samper et al., 2007] which encompass the predominantly normal Gauss period (fig. 2). This anomaly extends offshore towards the north. The magnetic signal of the Septentrional chain (Matuyama period [Samper et al., 2007]) displays a slight north to south evolution from positive to negative anomalies.

Along the Septentrional and Axial chains, several short wavelength anomalies are associated with small well defined volcanic edifices (domes or pitons). Some of them are SW-NE oriented in a direction perpendicular to the arc, as previously described by Le Mouël et al. [1979]. To the south of the Bouillante-Capesterre fault [Feuillet et al., 2001] the magnetic map can be divided in two main domains. The western area is marked by negative anomalies, whereas the eastern one associated with the recent volcanic activity (La Soufrière) evidences a strong positive signal. In the continuation, the Monts Caraïbes are associated with a slightly positive signal. These latter include the andesitic volcanoes of the recent complex and extend offshore to the south towards Les Saintes. On the contrary, the eastern coast of Basse-Terre (erosional plain) and the isthmus between both islands do not display any well defined magnetic anomalies. As in the case of the gravity signal, the magnetic structure of Grande-Terre is easier to understand, with three main positive anomalies, to the north of Saint Jacques Plateau, along the eastern coast of Grippon plain and in the western part of the Grands Fonds area. In these areas, as for Marie Galante and La Désirade islands, the map computed at 500 m a.s.l. also evidences positive anomalies implying shallow sources. As already suggested by the previous magnetic studies [Le Mouël et al., 1979] the Grands Fonds anomaly, at least, could be interpreted as reflecting a doming of the volcanic basement [Bouysse and Garrabé, 1984]. Coherent gravity and magnetic signals (fig. 6)

FIG. 5. – a) Magnetic anomaly map computed at an altitude of 1800 m a.s.l.; b) reduced to the Pole (RTP; declination=0°; inclination=29.7°) anomaly map computed at an altitude of 1800 m. The main regional faults are located, MBS: Montserrat-Bouillante-Les Saintes fault system; BC: Bouillante-Capesterre fault. Bull. Soc. géol. Fr., 2013, no 1-2

The newly compiled geophysical data evidence some clear coherencies between the gravity and magnetic signals. Locally on Guadeloupe islands, at the scale of Grande-Terre, three main areas (Saint Jacques plateau, Grands Fonds and Saint François areas) depart from the general low amplitude negative signal, quite coherently imaged in gravity and magnetic data. Saint Jacques anomaly is associated with the N050oE normal faults documented by Feuillet et al. [2001] at the western termination of the 5000 m deep, N050o to N130oE striking La Désirade valley [Feuillet et al., 2010]. The other well-defined anomalies do not evidence any clear link with the faults recognized in this area. All of them evidence low amplitude and short wavelength gravity and magnetic positive anomalies. Such characteristics suggest shallow sources and argue for an up-rise of the volcanic basement beneath Saint Jacques plateau, Grands Fonds and Saint François areas. On Basse-Terre, a N160oE gravity and magnetic axis crosses coherently the volcanic edifices, following the MBS fault system direction along the island. To the north, this latter separates the high amplitude positive gravity domain of the Septentrional chain from the eastern negative one associated with the erosional plain. To the south it marks the transition between the high positive gravity and magnetic signals associated with the recent volcanic complex from the western coast of the island, along the Ty fault mapped by Feuillet et al. [2001], and already documented in gravity by Gunawan [2005]. On the contrary Marie Galante basin is associated with E-W geophysical signals. As in the case of La Désirade valley, Marie-Galante graben is characterized by V-shaped structures striking to the west [Feuillet et al., 2010].

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FIG. 5c. – Extract of the RTP map centered on the Guadeloupe archipelago. The main places discussed in text are located.

Figure 6 summarizes the main gravity and magnetic trends derived from the qualitative interpretation. Even if most of them are well distinct, we can propose complementarities between both geophysical signals in regard of the structure of the overall arc. To the NW of the map, the N-S magnetic en-echelons axes are parallel to the arc whereas the main E-W gravity anomalies are perpendicular to the arc direction. In addition, the northeastern gravity trends along the Outer arc define a range of orientations from N-S to about N130oE. In this area, the ruptures within the en-echelon magnetic trends allow defining five major deformation axes. These latter will primarily affects the Atlantic convergent plate and secondarily deforms the upper Caribbean plate and the accretion prism, as documented in bathymetry and on-shore regional morphology [Feuillet et al., 2001 and 2010; Kopp et al., 2011]. Orientations of these axes progressively turn from about N020 oE to the north until reaching the N130 oE direction to the south. As an example, both La Désirade valley and Marie Galante basin match two of this regional gravity and magnetic axes of deformation. Their prolongation towards the west mark the expression in surface of this fan-shaped folding, coherently with active deformations already evidenced on- and offshore. MODELING OF THE MAIN GEOPHYSICAL ANOMALIES AND CONSTRAINTS Modeling approach We have carried out different modeling approaches in order to define the gravity and magnetic structure of the area of

interest. 3D inversions have been computed using GRAV3D and MAG3D software [Li and Oldenburg, 1998]. These models have the advantage to take into account 3D effects but they provide very smooth and unconstrained models of the magnetization and density distribution, which we used only as information for forward 2D models. They will not be discussed in this study. In the following, we present 2D models, which have been built using GMSYS software (Geosoft) in order to define the first order gravity and magnetic structure of the Guadeloupe volcanic system. The models have been extended at both extremities of the profiles in order to avoid the edge effects even if they are low constrained at the land-sea transition. Such 2D approach allows more realistic modeling, constructed with more geological and geophysical constraints than the 3D inversions [Gailler and Lénat, 2010]. Geological constraints Density values in the models have been constrained using geophysical and geological observations. The homogeneous reduction density of 2.67 103 kg m-3 of the Bouguer map corresponds to an average density of the whole volcanic system. However, a large range of densities exists among the rocks of Guadeloupe island. From measurements on various samples from the Bouillante geothermal province and La Soufrière volcano (Gailler et al., in prep; tab. II), different types of formations have been modeled using realistic densities ranging from 1.6 to 3.2 10 3 kg m-3. For example, La Soufrière volcano formations, which macroscopically appear to be low density vesiculated material, have been Bull. Soc. géol. Fr., 2013, no 1-2

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FIG. 6. – Comparison between the main gravity and magnetic features from the regional qualitative interpretation.

measured at about 1.6 10 3 kg m-3. Conversely, massive andesitic lava flows sampled in Bouillante area have been measured between 2.6 and 2.8 10 3 kg m-3 (tab. II). Offshore, apart from the bathymetry and seismic data, very few constraints exist to infer the density of the submarine flanks. We have therefore kept the same range of densities for the submarine continuation of our models. In regard of the magnetization values, a few data exist over Guadeloupe island except a paleomagnetic study carried out by Carlut et al. [2000] with an averaged value of 10 A.m-1 for most of the study samples. This value is of the same order of magnitude than those proposed in similar contexts such as La Martinique island [Salomé and Meynadier, 2004]. In addition, the available dating [Samper et al., 2007] enable to distinguish the rocks emplaced during the Brunhes normal-polarity, those erupted before the Matuyama reverse-polarity epoch (0.78 Ma) or during the Gauss normal-polarity period (2.6-3.6 Ma). As summarized on figure 2, the Basal complex should be associated with normally magnetized formations (Gauss period), the Septentrional chain with reversely magnetized formations (Matuyama period), the Axial chain with both normally and reversely magnetized formations (Matuyama and Brunhes periods), the Monts Caraïbes and the actual volcanic complexes with normally magnetized formations (Brunhes period). At the scale of Grande-Terre, the old volcanic basement should be associated with reversely magnetized formations covered by limestone platforms, mainly reversely, and more locally normally magnetized. Offshore, since the magnetization of the underwater formations cannot be precisely constrained, we have kept the same range of magnetization of ± 10 A m-1 for the overall models. Bull. Soc. géol. Fr., 2013, no 1-2

Geophysical constraints Another useful constrain to the interpretation of the internal structure of the system is provided by three models deduced from seismic refraction profiles acquired over Guadeloupe island (fig. 7a [Dorel et al., 1979]). Along these profiles, an interpretation in three seismic layers was proposed (fig. 7b). The shallow layer was interpreted as consolidated or semi-consolidated materials, such as sediments, pyroclastic and detritic products. The intermediate layer presents a range of seismic velocities, which could be attributed to several rocks, variety from consolidated sediments to volcanic formations and some metamorphic rocks. The nature of this layer is however still discussed. The deep layer was assimilated as the oceanic crust. Seismic wave velocity and density are two correlated parameters and various relationships have been defined in the literature. Here we have used the formula proposed by Gebrande et al. [1982] which describes the following quantitative relationship between longitudinal wave velocity VP (in km s-1) and density r (in 103 kg m-3) for volcanic rocks: VP = 2.81␳ - 2.37 ± 0.18 The equivalences between seismic velocities and densities as well as the thickness of the layers inferred from the seismic profiles are summarized in the table III. Using this information, the internal structure imaged by the seismic profiles was converted into density layers in order to reproduce the observed gravity signal. The shallower layer is modeled with a density of 2.0 10 3 kg -3, the intermediate one with a density of 2.4 10 3 kg m-3 and the deeper one with a higher density of 3.0 103 kg m-3.

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TABLE II. – Density parameters measured on various types of formations from the Bouillante geothermal province and La Soufrière. Location of referenced samples on the left. Bulk density: density of the solid part + density of the voids within the sample; True density: density of the solid part of the sample; Porosity: volume of the voids within the samples, reported as a percentage of the bulk volume of the sample.

QUANTITATIVE INTERPRETATION OF THE MAIN GEOPHYSICAL ANOMALIES We propose here a first quantitative interpretation of the main gravity and magnetic anomalies evidenced at the scale of the Guadeloupe island sensu stricto. Taking into account the available geological constraints, together with the seismic interpretations, the previously described 2D modeling approach has been carried out across Guadeloupe island and its land-sea transition, along three profiles (fig. 8, 9 and 10). The models have been built from the residual gravity anomaly, for which, as previously mentioned, the influence of the deep crustal structures was mostly removed. As a consequence, the base of our models has been fixed to –6 km in order to take into account the upper part of the oceanic crust at depth according to seismic refraction

studies [Dorel et al., 1979]. In order to strengthen the model, we have jointly modeled the RTP magnetic signal along the same profiles. Note that this latter has been modeled from the data upward continued at 3000 m a.s.l. in order to filter part of the short wavelength anomalies and to provide simpler regional models. Main structures of the Guadeloupe island (fig. 8 and 9) The first modeled cross section (fig. 8) corresponds to the refraction seismic profile C [Dorel et al., 1979; fig. 7], oriented E-W, and crossing Basse-Terre and Grande-Terre islands. The second also oriented E-W (fig. 9) is collocated with the refraction seismic profile A south of Basse-Terre [Dorel et al., 1979; fig. 7], and continues eastward across Marie Galante basin. This approach enables to check the

FIG. 7. – Location of the seismic velocity models across Guadeloupe Island and seismic velocity distribution in depth according to Dorel et al. [1979]. Bull. Soc. géol. Fr., 2013, no 1-2

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coherency between both methods at the scale of the overall island but also in its southeastern offshore continuation. To the first order and for both profiles, the long wavelength gravity signal is well reproduced by the simple three layers derived from the seismic refraction model [Dorel et al., 1979]. However, the thickness of the seismic layers has been slightly adjusted where necessary to satisfy the observed gravity signal. In our gravity models, the base of the upper layer lies between 1 and 2 km depth and the intermediate one at around 4 km in depth. Additionally, the short wavelength anomalies are fitted introducing some geologically realistic shallow density contrasts: the low resolution of the refraction seismic profiles could probably not detect the velocity variations associated with such shallow and local structures (Bitri, personnal communication). In more detail, for the northern profile (fig. 8), fit of the gravity signal requires denser formations in Basse-Terre volcanic edifice, especially over the Septentrional chain, which is associated with a denser layer (3.0 10 3 kg m-3) reaching a maximum thickness of about 400 m in its central part. Conversely lower density formations (~1.6 10 3 kg m-3) are modeled over Grande-Terre area. These local refinements of the model are in agreement with the nature of the outcropping formations: to the west, massive lava flows often constitute the volcanic relief, whereas to the east, Grande-Terre is a low density carbonated platform. For the southern profile (fig. 9), the short wavelength negative anomaly observed at the scale of La Soufrière is well reproduced by modeling a 500 to 1000 m thick low density formation (1.8 103 kg m-3) associated with the active volcano. This contrast agrees with the low density formations measured in this area (tab. II). In the magnetic models, we make the assumption that the three main seismic and gravity layers are also associated with different magnetization values. Since we do not have any constraints to define the geometry of such magnetic layers along the overall profiles, we have considered that each of them is continuous following the same geometry than the gravity models. A reversely magnetized volcanic basement (-6 A m-1) is associated with the deep dense seismic layer. It is covered by normally magnetized formations (6 A m-1; intermediate seismic layer) from the Gauss period associated with the Basal complex, the erosional plain of Basse-Terre and the limestone platforms over Grande-Terre (1 to 1.5 km thick in average). For the southern profile, the shallow seismic layer is divided in two main magnetic layers. The deeper one is modeled as reversely magnetized (-6 A m-1) in order to account for the Septentrional and the

ancient part of the Axial chain, Matuyama formations. The shallower one is considered as normally magnetized (6 A m-1) for the recent formations of the Axial chain together with the active complex of La Soufrière-Grande Découverte area. Such a magnetization distribution allows reproducing the long wavelength magnetic signal. Following the same rationale than for the gravity models, and with compatible geometry if possible, the short wavelength anomalies are accounted for considering local shallow magnetic heterogeneities (200 m to 1 km thick in average): the Septentrional chain (– 1 and 6 A m-1), the erosional plain (– 2 A m-1), the Grands Fonds (– 10 A m-1) and the eastern plateaus areas (4 A m-1) (fig. 8). The western contrasts of magnetization modeled over the Septentrional chain agree with the presence of massive lava flows in this area. On the contrary, the erosional plain to the east of Basse-Terre is marked by slightly reversely magnetized formations, which can be attributed to its detritic nature. The shallow magnetization over Grande-Terre most probably related to the detritic magnetization of the limestone platforms. The active volcano is modeled as a demagnetized structure (0 A m -1), which is probably reliable to the volcanic activity and the presence of the hydrothermal system generating the demagnetization [Lénat and Bachèlery, 1990; Lénat et al., 2000; Peltier, 2007; Peltier et al., 2009]. Both models display an overall good consistency between the gravity and magnetic structures. The three geophysical methods (seismic, gravity and magnetic) argue for a depressed structure under Guadeloupe island. Along the northern profile (fig. 8), it extends over about 10 km from the centre of the volcanic island (with its western limit modeled just below the Septentrional chain), to the isthmus between Basse-Terre and Grande-Terre to the east. Along the southern profile (fig. 9), a comparable depressed structure is modeled extending from the Ty fault [Feuillet et al., 2002] which cross-cuts La Soufrière, to about 40 km to the east, with a deepening of the depression just below Marie Galante basin. Note that the western limit of this depressed structure is offset from about 5 km eastward compared to the depression of the northern profile. A similar structure has been evidenced based on bathymetric and seismic reflection data [Feuillet et al., 2010] offshore, to the north, between Basse-Terre and Montserrat. It is limited by the MBS fault system to the west and extends over about 25 km to the east, with its eastern boundary in the offshore continuation of Basse-Terre eastern coast. This depressed structure, compared to the one we model along the profile C is offset from about 20 km to the west. In

TABLE III. – Equivalence between seismic velocity and density [Gebrande et al., 1982] for the three layers evidenced by Dorel et al. [1979]. The thickness or the depth of these layers is derived from the seismic refraction profiles. The densities values chosen for the modelled formations are also specified.

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agreement with Feuillet et al. [2010] this depressed structure appears in its entirety as a half-graben, with a well-defined western scarp and a more gradual up-rise towards the east. In addition, studies carried out south of Marie Galante have evidenced a comparable depressed structure in the southeastern continuation of the island. Main structures of Basse-Terre (fig. 10) The third modeled profile is oriented N-S along Basse-Terre island. It crosses the main anomalies evidenced in this area,

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i.e. the Basal complex and Septentrional chain, La Soufrière volcano and Monts Caraïbes anomalies (fig. 10). As for the two E-W profiles, the long wavelength gravity signal is fairly consistent with the overall geometry of the three seismic layers, where the model crosscuts the two seismic lines [Dorel et al., 1979]. The short wavelengths have been modeled by some irregularities along the interfaces, or by adding local shallow density contrasts imputable to geological formations recognized at the surface (fig. 10a). Even if the magnetic structure is more difficult to assess (fig. 10b) the model has been built coherently with the other profiles,

(a)

(b) FIG. 8. – a) 2D gravity model along the seismic velocity profile C across Basse-Terre and Grande-Terre islands [Dorel et al., 1979] located on the left on the Bouguer anomaly map. The main layers have been defined according to the seismic velocity distribution. Density values in 103 kg m-3; b) 2D magnetic model along the same profile located on the left on the RTP magnetic anomaly map. The transition between the normally and reversely magnetized layers has been defined according to the depth of the shallow gravity layer modeled in (a). Magnetization values in A m-1. Bull. Soc. géol. Fr., 2013, no 1-2

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(a)

(b) FIG. 9. – a) 2D gravity model along the E-W profile across Basse-Terre island and Marie Galante basin located on the left on the Bouguer anomaly map. This profile is collocated with the seismic profile A from Dorel et al. [1979] across Basse-Terre island. The main layers have been defined according to the seismic velocity distribution [Dorel et al., 1979]; b) 2D magnetic model along the same profile located on the RTP magnetic anomaly map. The transition between the normally and reversely magnetized layers has been defined according to the depth of the shallow gravity layer modeled in (a). Magnetization values in A m-1. Bull. Soc. géol. Fr., 2013, no 1-2

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considering a superimposition of normally and reversely magnetized layers, accounting for the different complexes of Basse-Terre (fig. 2). Note that all the three modeled profiles are consistent both in terms of depths and thicknesses at their intersections. In more detail, the Basal complex is modeled with normally magnetized formations (4 A m-1), consistent with the ages measured in surface, but without any density contrast (2.67 10 3 kg m-3). Even if the lower part of the Septentrional chain is reversely magnetized (– 1 A m-1), its upper part is modeled with dense (3.0 103 kg m-3) and normally magnetized formations (6 A m-1; 200 to 400 m thick) to fit the observed gravity and magnetic signals. The shallow dense contrast (2.8 103 kg m-3) modeled over the Monts Caraïbes is included in a normally magnetized structure (6 A m-1) in agreement with the presence of recent massive lava flows in this area [Samper et al., 2007]. Conversely, as evidenced in figure 10a and coherently imaged along the southern profile (fig. 9a), La Soufrière is locally marked by low density (1.8 103 kg m-3) in agreement with the highly vesiculated volcano formations. It is also associated with demagnetized material attributed to the volcanic activity and the hydrothermal alteration recognized in this area. A E-W elongated graben bordered by the Bouillante-Capesterre fault has been proposed by Feuillet et al. [2002] in the continuation of the E-W Marie Galante graben, under the active volcano as controlling the present volcanic activity. However, such structure could not be confirmed by our models; possibly because of a too low resolution of the geophysical data which document the predominance of a NNW depressed structure modeled along our E-W profiles. DISCUSSION AND CONCLUSIONS (fig. 11) Previous studies of the submarine environment of Guadeloupe island and other volcanic islands have mostly focused on the surface products, using bathymetry, acoustic images and sampling along the Lesser Antilles arc [Deplus et al., 2001; Le Friant et al., 2002; Le Friant and Boudon, 2003; Le Friant et al., 2004]. The original aspect of the work presented here is to use gravity and magnetic signatures to extend the investigation at depth. Both such data, available on land but also offshore at the scale of the Lesser Antilles arc, have been compiled and processed in order to provide regional Bouguer and RTP magnetic anomaly maps at the highest achievable resolution to date. This large scale geophysical approach does not allow unambiguous interpretations of the volcano-tectonic features because of the complexity of the arc, especially at the scale of Guadeloupe archipelago. However, it provides a regional image of the overall system and, using other available geological and geophysical information both on- and offshore, raises new hypothesis and questions concerning the tectonics and deep structure of the arc. Following an imbricate dual two scale analysis of the main gravity and magnetic anomalies, we propose a general structural scheme of the Lesser Antilles arc, and more locally of Guadeloupe island (fig. 11). To the east of the study area, the Outer arc is marked by three long wavelength positive gravity trends following a main regional N-S orientation. These features are interpreted, at least along the Karukera spur, as an up-rise of the volcanic basement beneath a thinned sedimentary coverage

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in agreement with seismic studies carried out at the scale of the Lesser Antilles arc [Evain et al., 2011; Kopp et al., 2011; Ruiz et al., 2011]. It would correspond to the eastern margin of the volcanic system outcropping on La Désirade island and also punctually along the scarp of Karukera spur, as also suggested by the local gravity and magnetic anomalies. In this area, from the north of the outer arc to the south of the Tiburon ridge, the newly compiled RTP map evidences long wavelength en-echelon anomalies which are mainly N-S oriented (fig. 6). Even if these magnetic trends integrate shallow effects, their long wavelengths primarily suggest deep structures, and two main hypotheses can be proposed in regard of their origin. They could be related either to deep structures within the Caribbean upper plate, or more probably to the subducting Atlantic oceanic plate compatible with a normally magnetized stripe if the Atlantic oceanic floor, as recognized further to the NE. In more detail, the Outer arc main gravity long wavelength anomalies are disrupted by short wavelength axes which cover a range of orientations from about N020oE to the north to N130 oE to the south. These fan-shaped gravity axes are compatible with five main discontinuities disrupting the N-S magnetic trends into an en echelon magnetic pattern. These axes are interpreted as large fracture zones affecting primarily the subducting Atlantic oceanic plate and secondary deforming the accretion prism and the Caribbean upper plate. They are consistent with the overall deformation of the Outer and Central arcs as documented in bathymetry with the major structures of Antigua valley, Bertrand Falmouth spur and La Désirade valley, and also in the onshore regional morphology [Feuillet et al., 2001 and 2010; Kopp et al., 2011]. A transition occurs within both gravity and magnetic signals to the south of the Tiburon ridge, with the convergence of both arcs in a single one southwards, and also the change of strike of the Benioff zone beneath the Lesser Antilles at about 14oN [Wadge and Shepherd, 1984]. This zone, SE of Martinique island is also where the buried southern Ste Lucia ridge has been documented [Andreieff et al., 1987]. Since the N130 oE direction of these ridges is well represented in our geophysical data, each of them may have a major impact on the arc evolution, from a tectonic but also magmatic point of view. As proposed by Andreieff et al. [1987], they could play a locking role in the subduction process because of their morphology direction transverse to the arc. The complex regional deformation and faulting system of the NE area is certainly in relation with the subduction process with probable outstanding influence of the convergent N130oE Atlantic oceanic ridges. Conversely, the central domain of the arc (ancient arc) is characterized by a main negative gravity area, which can be divided in two parts. The first, central, one is oriented parallel to the arc following a N160oE direction, and bordered to the northwest by the MBS fault system. It could be interpreted as the normal arc-parallel central area of subsidence of the upper oceanic plate, where present to early Miocene deformations have been reported [Lardeaux et al., 2013; Münch et al., 2013]. The second large gravity negative anomaly is located east of Dominique island, parallel to the N130 oE Tiburon ridge direction. Its significance is highlighted by our geophysical modeling performed on Guadeloupe island and discussed below. Bull. Soc. géol. Fr., 2013, no 1-2

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At the scale of Guadeloupe island, based on the seismic velocity models proposed by Dorel et al. [1979] joint gravity and magnetic models allow addressing the internal structure of the island. The long wavelength gravity and magnetic signals are well accounted for considering three main layers with coherent geophysical parameters. The short wavelength anomalies require shallow and localized formations with magnetization and density contrasts. Most of them can be related to geological formations recognized

in the field (weathered or hydrothermalized formations, andesitic lava flows …). Grande-Terre internal structure is more homogeneous with local structures attributed to up-rises of the volcanic basement below the carbonated platform [Bouysse and Garrabé, 1984], as also suggested for Marie Galante anomalies [Kopp et al., 2011]. The modeled E-W profiles extending across Basse-Terre, Grande-Terre islands and Marie Galante basin, coherently image a NNW depressed structure bounded to the west by a

(a)

(b) Fig. 10. – a) 2D gravity model along the N-S profile located on the left across Basse-Terre island on the Bouguer anomaly map. The main layers have been defined according to the seismic velocity distribution [Dorel et al., 1979]; b) 2D magnetic model along the same profile located on the RTP magnetic anomaly map. The transition between the normally and reversely magnetized layers has been defined according to the depth of the shallow gravity layer modeled in (a). Magnetization values in A m-1. Bull. Soc. géol. Fr., 2013, no 1-2

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well defined scarp. This latter cross-cuts La Soufrière to the south and the Septentrional chain to the north along a N160oE direction, parallel to the MBS fault system in surface. To the east, the limit of the depression is located on the isthmus of Guadeloupe, along the Grands Fonds area. It strikingly agrees with the structures imaged in seismic and bathymetric data [Feuillet et al., 2010], offshore Guadeloupe archipelago, along the MBS towards Montserrat to the north. It is characterized by a half-graben geometry, with its western scarp well-defined along the Montserrat-Bouillante fault system. Following the hypothesis of an extensive system associated with the subduction zone in the central arc, we interpret this depressed corridor as a subsidence zone. Its SW fault scarp may be the present-day active front of deformation within the Caribbean plate. To the south of Guadeloupe, this front of deformation marked by the MBS fault system, presents an inflexion to N140 oE around Les Saintes, and perfectly outlines the SW limit of a negative gravity diverticulum striking N130 oE further to the SE, parallel to Tiburon ridge. Within this negative zone, evidences to actual to early Miocene subsidence have been reported to the SE of Marie Galante island. This feature argues in favor of a secondary subsident area transverse to the arc, along the N130 oE direction, parallel to the Atlantic ridges reported in this area. These interferences between two directions of subsidence within the arc, together with the fan-shaped folding of the outer arc obviously related to deep-seated crustal structures, sheds new lights on the sinistral transtensional kinematics reported on the MBS fault system [Thinon et al., 2010; Calcagno et al., 2011;

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Mathieu et al., 2011] and more widely in the active front of deformation of the Caribbean plate, as defined in our study (fig. 11). In conclusion, this work provides a regional interpretation of the structure of the Lesser Antilles arc, consistent with a more local assessment of the internal structure of Guadeloupe island sensu stricto. The approach presented here should be generalized and detailed at both scales. At the regional scale, it provides a unique dataset to address the structure and evolution of the arc as a whole. It also brings new additional constrains to understand this complex volcano-tectonic system. The detailed analysis of the offshore magnetic lineaments should be further investigated to understand the inheritance of the lithospheric structure in the island evolution. The role of the main bathymetric ridges within the subduction remains to understand. In addition, the seismic interpretation combined with the geophysical models could be used to evaluate the thickness of the sediments lying on the volcanic basement offshore. The models could then be corrected from this parameter and consequently help to better constrain the internal structure of the arc. At the scale of the Guadeloupe island, each area of interest evidenced in this study should be surveyed and analyzed in more detail in order to ascertain the origin and the nature of the different formations at depth. Other particular topics could be addressed by local gravity and magnetic studies, such as, for instance, the one carried out over the Bouillante area, in order to better constrain the fundamentals of this geothermal province (Gailler et al., in prep).

FIG. 11. – Interpretative structural and volcano-tectonic scheme of the Lesser Antilles arc and the Guadeloupe islands based on the recognized structures (e.g. [Bouysse et al., 1988; Feuillet et al., 2001; Calcagno et al., 2011; Evain et al., 2011]) and the geophysical (gravity and magnetic) features (this study). Bull. Soc. géol. Fr., 2013, no 1-2

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Acknowledgements. – The study presented here has been carried out in the framework of a “Carnot Institute” labeled project with the financial support of the ANR, and performed within the BRGM group. This work has benefited from data acquired by numerous scientific projects. We thank the captains and crew, and their scientific leaders: P. Bouysse (Arcante, 1980), C. Deplus (Aguadomar, 1998), U. Send (Move, 2002), P. Guennoc

(Bathybou98, 1998), I. Thinon (Geoberyx, 2003), J.-F. Lebrun (Kashallow, 2009 and 2011, supported by the FEDER and the Guadeloupe region). We greatly thank the editors and the reviewers, Laurent Michon and an anonymous reviewer, for their insightful comments which provokes a deep improvement of an earlier version of the manuscript.

References

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